(1) Field of the Invention
The present invention relates to the manufacture of ultra large scale integrated (ULSI) circuit chips in general, and in particular, to forming interconnects in a semiconductor substrate by using a dual damascene interconnect process with borderless contact.
(2) Description of the Related Art
As the very large and ultra large scale integration (VLSI and ULSI) of circuits progresses to even smaller feature sizes less than half micron, one of the most important challenges to be overcome is the level-to-level alignment in lithography, especially that of complex structures of metal line and contact interconnects. The packing density of circuits is limited to a large extent by how closely the interconnect metal between circuits can be formed without encroaching on each other. As is well known in the field, the limits are dictated by design rules that govern the separation of one level of contact from another, and by design rules for nesting tolerance or for borders used around contacts. The present invention discloses a method for forming interconnects without borders, taking advantage of a dual damascene process. The method is equally applicable to either a metal plug formed through a contact hole over a device in a substrate, or through a via hole connecting two metal layers at different levels in a substrate. It will be known to those skilled in the art that contacts refer to an interconnect which interconnects a source-drain device region, salicide of polysilicon to metal, while vias refer to an interconnect which connects metal to metal.
The effect on packing density of borders around contacts is well illustrated by S. M. Sze, et al., in an article published in ULSI Technology. FIGS. 1a, 2c in the drawings, adapted from Sze, show a gain of more than 62% in the packing area in going from a contact having borders to no borders. FIG. 1a shows fully bordered, staggered vias (23) and (33) formed between metal layers (20) and (30. Metal layers have been patterned to form metal lines (27) at the lower level and metal lines (37) on the upper level. Single and double primed reference numerals (27) and (37) refer to other metal lines at the respective levels (20) and (30), respectively. Via (33) on the upper level has border (31) and via (23) on the lower level has its border (21). Ideally, the pitch between metal lines such as (35) for the upper metal lines and (25) for the lower metal lines, is determined by the minimum line and space dimensions that can be patterned using the most recent advances in lithographic techniques. In practice, line pitch is also limited by the via size and the underlying metal pad size forming the border around the via, such as (31) and (21). It will be known by those skilled in the art that a border around a via is needed, for otherwise, grooves would be etched into the underlying insulating layer during the via-etch step, thus causing a thinning of the next level of metal deposited over the via. The minimum dimension by which the metal pad must frame the via, that is, form a border, is dependent on the misalignment tolerances of the lithography step.
Furthermore, the slope of the via wall must be taken into account when determining the minimum pitch between vias. Sloped walls are needed so that the vias can be filled more easily with metal, and without any voids inside the via holes. Also, appropriate slope is needed for adequate metal coverage over the step of the edge of the via hole when physical vapor deposition is employed. The step coverage is in turn dependent upon the aspect ratio, that is, depth over the width of the via hole. A cross-sectional view of vias (55) and (65) with sloped walls (57) and (67), respectively, is shown in FIG. 1b. It will be noted that the more is the slope of the via wall, the larger the border must be for the metal pad over the lower via to insure full coverage of the via.
It is also noted that the vias of FIG. 1a and FIG. 1b are formed laterally with respect to each other. That is, they are staggered rather than being stacked on top of one another as shown in FIG. 1c. The pitch between staggered vias can be reduced if the borders around the vias can also be reduced. The borders can be reduced if the slope of the walls can be reduced. The slope can be reduced if the holes can be filled properly with walls approaching vertical orientation. As is known in the art, forming metal plugs, such as tungsten plugs, in via holes separate from forming metal lines makes vertical vias possible. Plug forming methods are advantageous also in filling contact or via holes of different cross-sectional areas, though they may not fill the openings up to the top. This is shown by reference numerals (75) and (85) in FIG. 1c where vias (73) and (83) have more steeply sloped walls, and they span, respectively, insulation layers (70) and (80).
Vias with vertical walls, and with no borders, can also be made, as disclosed later in this invention and as depicted in FIG. 2a. In FIG. 2a, vertical vias (93) and (103) are formed in insulating layers (90) and (100), respectively, connecting two metal layers (97) and (107). Thus, the minimum distance, such as (25) and (35) in FIG. 2b, between adjacent metal lines, is reduced since the vias have no slope. Secondly, the borders are no longer needed, since plugs (23), (33) in the completely filled vias provide ample overetch protection to underlying metal structures without mask coverage, as it will be appreciated by those skilled in the art. Even with borderless vias only, then, there is substantial reduction in the area occupied by these vias as shown in FIG. 2b in comparison with vias with borders of FIG. 1a. Hence, the pitch (35') between adjacent lines (37) and (37') can be reduced. The line pitch can be further reduced if the vias are stacked on top of one another as shown in FIG. 2c where upper vias (23), (23') and (23") are stacked on top of lower vias (33), (33') and (33"), thus significantly reducing upper metal line pitch from (35) to (35"), and lower metal line pitch from (25) to (25"). Hence, significant gains in packing density can be achieved with stacked and borderless contacts or vias.
However, the practice of stacking contacts and borderless contacts is still in its infancy, as observed by Sze in the earlier Reference. Conventionally, the metal layers and the interconnecting layers are formed separately, and serially. First, a first blanket metal is deposited on a first insulating layer and electrical lines are formed by subtractive etching of the metal through a first mask. A second insulating layer is formed over the first metallized layer, and the second insulating layer is patterned with contact or via holes using a second mask. The holes are then filled with metal, thus forming metal columns, or plugs, contacting the first metal layer. A second blanket metal layer is formed over the second insulating layer containing the columnar plugs which now connect the upper second metal layer with the lower first metal layer. The second metal layer is next patterned with another mask to form a set of new metal lines, and the process is repeated as many times as it is needed to fabricate a semiconductor substrate. With this conventional process, lithographic alignment tolerances must be held very tight in order to form reliable borderless contacts or vias.
On the other hand, forming plugs and metal lines simultaneously, as in the modified dual damascene process disclosed in this invention, some of the misalignment problems can be overcome. In the standard dual damascene process the insulating layer is coated with a resist material which is exposed to a first mask with the image pattern of the via opening and the pattern is anisotropically etched in the upper half of the insulating layer. After removal of the patterned resist material, the insulating layer is coated with another resist material which is exposed to a second mask with image pattern of the conductive lines in alignment with the via openings. In anisotropically etching the openings for the conductive line in the upper half of the insulating material, the via openings already present in the upper half are simultaneously etched in the lower half of the insulating material. After the etching is complete, both the vias and grooves are filled with metal. Though the alignment of contact or via holes with respect to the metal lines is improved, the standard dual damascene is still difficult to practice in forming borderless contacts and vias for metal pitches of sub-half micron, and with copper metal.
Some of the difficulties in forming borderless contacts have been addressed in prior art. For example, Huang, et al., in U.S. Pat. No. 5,654,589 use a landing pad both as a local interconnect and borderless contact. Huang teaches the forming of Ti/TiN stack interconnect structures that can be used as local interconnects and contact landing pads on the same level. The contact landing pads facilitate the use of borderless contact approach. In U.S. Pat. No. 5,466,636, Cronin, et al., show a method of forming borderless contacts using a removable mandrel. Zettler, et al., (U.S. Pat. No. 5,422,309) teach the forming of a metallization level having contacts and interconnects connecting the contacts. Shoda of U.S. Pat. No. 5,529,953 discloses a method of forming studs and interconnects in a dual damascene structure, while Chung, et al., of U.S. Pat. No. 5,604,156 teach a dual damascene process for forming a wiring. These all differ from the present invention where a method of forming stacked, borderless contacts and vias is disclosed using a modified dual damascene process.